1. Field of the Invention
The invention relates to a synchronization method for the transmission, on an asynchronous channel, of a series of pictures encoded by means of a variable length code, and a device for the implementation of this method. Such a method enables, for example, the transmission of digital video pictures on an asynchronous channel distributing these pictures to subscribers.
In the future, the distribution of video pictures will be carried out in digital form, by means of encoding methods providing a very high reduction in the information data rate, in particular methods of encoding by blocks of picture elements implementing a two-dimensional transformation such as the cosine transformation; and implementing a variable length encoding such as Huffmann encoding. The synchronization of the channel with the sampling frequency of the video pictures is not possible in practice as there are numerous sources of video signals which are independent from each other and which are independent from the transmission channel. It is therefore necessary to use a synchronization method enabling, on decoding, the recovery of the video sampling frequency and enabling the restitution of the pictures while limiting, as much as possible, the degradations due to transmission errors affecting the synchronization signals.
Furthermore, it is necessary to be able to distinguish the encoded data corresponding to each picture and the encoded data corresponding to each block within a picture.
The encoded data corresponding to each block of picture elements can be distinguished by means of separators called inter-block separators, and the encoded data corresponding to each picture can be distinguished by means of separators called picture separators, but a consequence of the use of variable length codes is that these separators do not have a constant position in the flow of binary data transmitted and it is not therefore possible to recover the sampling rate of the video pictures using a method of the filtering type. A first object of the invention is therefore to propose a synchronization method enabling the recovery of the sampling frequency of the video pictures, and enabling the various types of encoded data to be distinguished.
2. Description of the Prior Art
High performance encoding methods generally include a differential encoding stage for the blocks of picture elements whose brightness and colour differences have varied little with respect to the similar block in the immediately previous picture. This differential encoding stage enables a high reduction in the quantity of information to be transmitted but has the disadvantage of making a transmission error affecting one block catastrophic. In effect, if a series of similar blocks in the series of pictures is transmitted using this differential encoding and if an error affects the first block of the series, this entire series of similar blocks will be restituted erroneously. Furthermore, the absence of detection of an inter-block separator can affect the position of all of the blocks which follow and can therefore cause a shifting of the blocks of the restituted picture. And, if the following pictures are encoded using a differential encoding referring to the erroneous picture, they also will be restituted erroneously. Another object of the invention is therefore to propose a synchronization method avoiding the consequences of a transmission error in the synchronization signals propagating from one block to another in a same restituted picture and propagating in the series of restituted pictures.